The present invention relates to a pneumatic radial tire. More particularly, the invention relates to such a pneumatic radial tire in which end portions of a belt layer disposed on a carcass layer in a crown portion of the tire are covered with a belt covering reinforcing layer including fiber cords and which has improved high speed durability and tire uniformity.
Known pneumatic radial tires include such ones in which in a crown portion of the tire, at least two high stiffness belt layers having a cord angle of 10.degree. to 30.degree. relative to the circumferential direction of the tire are disposed on a carcass layer having cords arranged in a radial direction of the tire. It is also known that in order to enhance the high-speed durability of radial tires of the above structure, a belt covering reinforcing layer having cords arranged substantially in the tire circumferential direction is laminated at least at each end portion of the belt layers: When the tires are run at a high speed, the belt covering reinforcing layer functions to prevent the belt layers from undergoing such a phenomenon in which end portions thereof tend to come up due to the effect of centrifugal force. Thus, the belt covering reinforcing layer is incorporated as above so as to suppress the shearing strain in end portions of belt layers, prevent the belt layer end portions for undergoing a separation rupture and enhance the high speed durability of tires.
For the fiber cord for use in or for the belt covering reinforcing layer, conventionally polyamide fibers are normally utilized in that they have a good bonding property toward or relative to rubber and also a sufficient heat resistance for standing the heat generated as a result of a shearing deformation in belt layer end portions. Then, in order to enhance the initial modulus of polyamide fiber cords, it is normally operated to subject the cords to a heat treatment in a condition of the cords being applied with an excessive tension. However, when so treated, the polyamide fiber cords undergo a considerable increase in their heat shrinkage, so that at the time of vulcanization of tires, they tend to undergo a considerable extent of contraction, whereby a problem is likely such that there becomes a dislocation phenomenon generated in a splice portion in the belt covering reinforcing layer. If this phenomenon occurs, then the belt layers become waved and/or their width becomes irregular in the vicinity of the splice portion of the belt covering reinforcing layer, whereby the tire uniformity tends to be considerably lowered.
Another example of useful fibers for the belt covering reinforcing layer is the class of polyester fibers which essentially have a higher initial modulus than the above considered polyamide fibers. With polyester fibers, however, their molecular structure is such that the functional groups which they contain are relatively limited, so that they have the shorcoming that their adhesive or bond affinity to rubber is relatively limited. Therefore, in making use of polyester fibers, conventionally it has been indispensable to operate a two-step treatment for enhancing the bonding property of the fibers, which comprises preparatively treating the fibers with an epoxy based adhesive or an isocyanate based adhesive and then treating with a mixture solution comprising a resorcinol-formalin primary condensate and a rubber latex (the so-called RFL). To have to operate such a two-step treatment for enhancement of the bonding property is disadvantageous in that it lowers the efficiency of the production of the belt covering reinforcing layer and eventually raise the tire production cost. Another difficulty with polyester fibers resides in that their chemical stability is relatively low, so that the ester bonds in the polymer chain are prone to be hydrolyzed with moisture and amine compounds to be generated from a vulcanization accelerator contained in rubber, and as a result of this, it tends to be difficult to obtain a high cord strength. Actually, belt covering reinforcing layers made with use of polyester fibers were found not to come up to comparable layers made with use of polyamide fibers, with respect to the high speed durability.